Generally, a dust proof mask is used to prevent the inhaling of dust through the respiratory channel of the human body in places where the air contains a lot of dust i.e., in places such as coal mines, ship building yards, steel and iron manufacturing plants, wheat milling houses, fiber manufacturing factories, cement manufacturing plants, spray coating places and the like. Such a dust-proof mask employs a filter on the front or side thereof, i.e., at the air the inhaling path so as to capture dust particles.
Such a dust proof mask filter should have a low resistance against the inhaling and exhaling, and should be high in the dust capturing efficiency, as well as low in the manufacturing cost.
Natural fibers such as cotton and wool, or chemical fibers such as nylon, polyester, acrylonitrile, polypropylene, polyethylene and like may be used for the filter. In view of the resistance against the inhaling and exhaling, and in view of the dust capturing efficiency, wool is known to be the best, and wool is widely used in a form of felt.
However, wool alone is not sufficient for the function, and therefore, many attempts have been made to improve the physical properties of this material through chemical and physical treatments in the manufacturing process.
It is known to use wool felts which are formed by stacking and pressing wool fibers of yarn. However, in such a felt filter, a homogeneity cannot be obtained, and further, it is made by a batch type production so that mass production would be difficult. Further, it is insufficient in the resistance against inhaling and exhaling, and in dust capturing efficiency.
In order to solve the above described problems, various attempts have been made. For example, an electrostatic agent which is in a powder form and which promotes the electrostatic phenomenon is spread on the felt. As disclosed in Korean Patent Publication No. 90-2091, styrenic resin is mixed with petrolic resin or with alkyl phenol resin in an amount of 1 to 50 weight %, and this mixture is dissolved in a solvent such as methylene chloride, benzene, or toluene so as to prepare an impregnating resin solution. This solution is impregnated into non-woven fabric of wool, and then, the non-woven fabric is allowed to pass through rollers, so that a proper level of impregnation would be maintained. Then the non-woven fabric of wool is dried in a drier, thereby completing the chemical treatment for inducing electrostatic charges in the non-woven fabric.
The reason why electrostatic force is utilized is that the charged dust particles are captured by the electrostatic forces of the dust-proof mask filter. Further, even in the case of non-charged dust particles, they are charged by the electrostatic field of the filter, so that they would be captured in the same manner. Thus without increasing the resistance against inhaling and exhaling, the dust particles can be effectively captured.
The filter which utilizes the electrostatic forces is called an "electrostatic filter" in the art, and is capable of capturing even tiny dust particles by the electrostatic force.
Generally, a non-charged air filter captures dust particles mainly through the inertia and diffusion out of the capturing mechanisms. However, as shown in FIG. 1, the capturing efficiency through the two functions becomes the lowest around a particle size of 0.3 microns, with the result that particles of the sub-micron region can scarcely be captured. On the other hand, the electrostatic filter exerts a coulomb force and an electrostatic induction force in addition to the above mentioned two functions, with the result that larger amounts of particles can be captured in an effective manner.
However, the chemical treatment or resin treatment requires a separate process, and the chemicals or the resin particles may be inhaled into the human body. Particularly, in the case of an over-treatment, the gaps between the fibers are filled with the chemical or the resin, thereby increasing the resistance against the inhaling and exhaling.
In order to overcome the above described disadvantages, Korean Patent Publication No. 90-3634 which was filed by the present applicant discloses as follows. That is, by utilizing the fact that some chemical resin film has charge induction property, thin propylene film is stacked after thinly slitting it, and non-woven fabrics are attached on the upper and lower faces of the film. These dust proof filters have been commercialized.
However, in this product, the gaps between the fibers can be non-uniform, and the bound state is weak. Therefore, this product has to be used overlapped with a third non-woven fabric.